The invention relates to a process for the production of dielectric layers in planar circuits on ceramic substrates.
Planar circuits on ceramic substrates are known in which several circuit levels are insulated from one another by dielectric layers. These dielectric layers have apertures that serve both to accept permanent conductive connections and for connecting pins to be passed through for connection with other components. Especially apertures of the latter kind situated above corresponding holes in the substrate are advantageously made to be conical, narrowing towards the substrate, so that the dielectric layer is not damaged when the connecting pins are introduced, and a reliable connection is enabled between the connecting pins and the conductors. The known photolithographic processes, in which a paste comprising a photoresist and a powder dielectric material are used, as a rule only yield apertures whose sidewalls, are almost vertical to the circuit level. The screen printing process, in contrast, always yields sidewalls that are more or less sloping, and usually also curved. The shape of these walls depends on a number of process parameters and is difficult to control without affecting other properties of the finished product.
A process for the photolithographic production of apertures in photoresist coats that have sloping sidewalls is known from European Patent 227 851. In this case, after image-wise exposure, the photoresist layer is post-exposed diffusely. The result of this is that the upper areas of the entire layer become at least somewhat soluble. The thickness of the resulting layer can, therefore, not be kept uniform without difficulty. The known process is also only used on pure photoresist layers in the micrometer range and cannot be transferred to layers of photoformable pastes that are more than 20 .mu.m thick.